Fairly large update this time, onto reasonings:

White:

Niblis of the Urn is low on pick and low on use. It has been rotated out in exchange for Fearless Fledgling, a more aggressive "flyer" for the same cost.

Auramancer rarely gets an enchantment back. While the dream of using Auramancer to loop something like Angelic Renewal or to reuse a powerhouse like Spectra Ward has happened, it's rare. For those who like that, Hanna is still available. The replacement, Canyon Jerboa, is better in a wider variety of situations.

Blue:

Everyone loves Hedron Crab. That being said, there isn't that much landfall support outside of green, and mill is mostly blue/black. Manic Scribe also mills 3 a turn, and ties into Delerium, which is more supported in black than landfall.

Narset's Reversal is a solid card... sometimes. It's certainly not bad, and has relevant plays. Midnight Clock is a mana rock for control decks that also refuels your hand in a minimum of 6 turns.

Writ of Passage does unblockable for a subset of creatures. Sower of Temptation also does unblockable (by making there be less blockers).

Black:

We needed more sac outlets, so in comes the old standby Carrion Feeder to replace Diregraf Ghoul. Nothing against a vanilla 1 mana 2/2, but Feeder is basically that in most circumstances.

Dark Salvation is a wonderful mana sink that rarely sinks more than 5 mana. Unfortunately, Zombies aren't majorly supported. Collective Brutality does -2/-2 sooner in the game and has alternate modes.

Massacre is a variant on the classic Damnation, casting for free under certain circumstances. Unfortunately, it's not fantastic outside of the free-cast. Extinction Event does better when cast for the full cost.

Red:

Goblin Bushwhacker is a finisher for the red aggro style builds, but without a consistent way to get early, it does less than is desired. Glorybringer is also a finisher for mono-red, and does its job very well.

Desperate Ravings's random discard is traded for the discard your hand drawback on Ox of Agonas, who also comes with a body.

Crimson Muckwader is traded for the faster Stromkirk Noble.

Green:

Sunblade Elf is switched for the more versatile Pelt Collector. Sunblade was fairly low pick due to the reliance on Plains and the lack of opportunities to utilize the activated ability. Collector should patch that up.

Atarka Beastbreaker is similar. 5 mana per pump is very doable, but is also a heavy investment. A mono-green Trygon Predator variant in Feline Sovereign does solid work on its own, but the anthem (even for a less supported tribe) should be the icing needed to draw more picks.

Multicolor:

Blightning is a good card for Rakdos Aggro. Specifically it's a good reward for already playing red/black. However, it doesn't pull you into red/black on its own. For that, we turn to Rakdos, the Showstopper. Big, beefy, relevant abilities, and the chance to wipe the board.

I love Gaze of Granite. It's a board wipe that cleans up. However, Sarulf, Realm Eater just does more for more decks. It's counters, midrange, and a boardwipe rolled into one.

What to say about Horizon Chimera and Winged Coatl. They're both solid cards that earn low pick slots. Neither do anything fantastic which isn't great. Ice-Fang Coatl was an option for an uplift, but that requires bringing in snow as a theme. Therefore, Kiora, Behemoth Beckoner and Zegana, Utopian Speaker are replacing those two.

As the sacrifice deck is moving into the Jund slice, the 3-color sacrifice card has moved from the outlet Butcher of the Horde to the payoff Korvold, Fae-Cursed King.

Colorless:

We continue to remove the fixed color color fixing, replacing the remaining 5 with 2 equipment, 2 lands, and a mana rock.

Equipment:
The classic Bonesplitter. Good for aggro.
The game winning Sword of Body and Mind.

Mana Rock:
Prismatic Lens, a "5 color" mana rock that costs 2. Other options included Sphere of the Suns and Coldsteel Heart, but Prism Lens won the day.

Lands:
Ash Barrens is 5 color fixing on the land slot that also helps supply Delerium and other grave fueled strategies.
Volrath's Stronghold isn't fixing, but it does support grave strategies. It's also part of the mini-cycle of lands that care about colors with Academy Ruins that may or may not be finished one day.
Not a swap for a mana rock, but Cinder Glade is exchanged for the manland Raging Ravine. More power to counters and midrange, here we come!